Vinyl monomer compositions which are polymerized in situ to form protective coatings are well known. For example, methyl methacrylate compositions have been widely studied as impregnates or sealers for concrete, and as binders for aggregate field polymer concrete compositions that are used in coating or patching portland cement based concrete. Concrete treated in this manner can be more resistant to abrasion and chemical spills than untreated concrete.
Vinyl monomers may be polymerized in bulk by free radical polymerization initiated by high energy radiation, particle beams or chemical sources of free radicals such as peroxides and hydro-peroxides. It is also well-known that free radical polymerization of vinyl monomers may be inhibited by oxygen. Oxygen inhibition on polymerization becomes particularly troublesome in surface coating compositions such as those used in protecting concrete surfaces. In situations involving compositions based on methyl methacrylate, the lack of surface curing is not particularly disadvantageous. Methyl methacrylate has significant vapor pressure, and uncured monomer on the surface of an otherwise polymerized composition volatizes relatively quickly, exposing the hard polymeric surface beneath.
However, when the uncured composition contains less volatile vinyl monomers, the surface of the composition which has been cured in bulk polymerization may be very slow to cure since the presence of oxygen inhibits surface curing. This results in a surface having such undesirable properties as tack and residual odor. Compositions containing less volatile monomers may be otherwise desirable because of enhanced chemical resistance, greater safety, reduced odor and lower cost.
A variety of techniques have been used in an attempt to resolve the problem presented by inhibition of polymerization by oxygen. For example, a polyvalent metal salt such as cobalt naphthenate may be included in the coating composition in order to catalyze the oxidative cure of the residual uncured monomer. Driers such as cobalt naphthenate are commonly used to promote thorough drying. Also, articles coated with vinyl monomer compositions can be cured under a blanket of nitrogen or other inert gas. In an alternative procedure to resolve the issue of the presence of oxygen, a second coating of a material such as paraffin which reduces oxygen penetration at the surface of applied vinyl monomer can also be used to ameliorate the situation.
None of these solutions to the problem arising from oxygen inhibition of surface cure has been totally satisfactory. There remains a significant need for vinyl monomer compositions which rapidly develop surface cure, especially in the case of compositions which contain relatively low volatile vinyl monomers.